1. Field of the Invention
The present invention relates to a wireless power transfer device and a wireless power transfer method of wireless power transfer via a power transmission coil provided in a power transmitter and a power receiving coil provided in a power receiver.
2. Description of Related Art
As methods of wireless power transfer, an electromagnetic induction type (several hundred kHz), electric or magnetic-field resonance type using transfer based on LC resonance through electric or magnetic field resonance, a microwave transmission-type using radio waves (several GHz), and a laser transmission-type using electromagnetic waves (light) in the visible radiation range are known. Among them, the electromagnetic induction type has already been used practically. Although this method is advantageous, for example, in that it can be realized with simple circuitry (a transformer), it also has the problem of a short power transmission distance.
Therefore, the electric or magnetic field resonance-type power transfer methods recently has been attracting attention, because of a ability of a short-distance transfer (up to 2 m). Among them, in the electric field resonance type method, when placing the hand or the like in a transfer path, a dielectric loss is caused, because the human body, which is a dielectric, absorbs energy as heat. In contrast, in the magnetic field resonance type method, the human body hardly absorbs energy and a dielectric loss thus can be avoided. From this viewpoint, the magnetic field resonance type method attracts an increasing attention.
FIG. 17 is a front view schematically showing an example of the configuration of a conventional power transfer device using magnetic field resonance. A power transmitter 1 includes a power transmission coil unit including a combination of a loop coil 3a and a power transmission coil 4a (operating as a resonance coil for transmitting power). A power receiver 2 includes a power receiving coil unit including a combination of a loop coil 3b and a power receiving coil 4b (operating as a resonance coil for receiving power). To the loop coil 3a of the power transmitter 1 is connected a high-frequency power driver 5, which converts the power of an AC power supply (AC 100 V) 6 into a high-frequency power capable of being transmitted. As a load to the loop coil 3b of the power receiver 2, for example, a rechargeable battery 8 is connected via a rectifier 7.
The loop coil 3a is a dielectric element that is excited by an electric signal supplied from the high-frequency power driver 5 and transfers the electric signal to the power transmission coil 4a by electromagnetic induction. The power transmission coil 4a generates a magnetic field based on the electric signal that has been output from the loop coil 3a. The magnetic field strength of the power transmission coil 4a is a maximum when the resonant frequency f0=1/{2π(LC)1/2} (L represents the inductance of the power transmission coil 4a on the power transmission side, and C represents the stray capacitance). The power supplied to the power transmission coil 4a is wirelessly transferred to the power receiving coil 4b by magnetic field resonance. The transferred power is transferred from the power receiving coil 4b to the loop coil 3b by electromagnetic induction, rectified by the rectifier 7, and supplied to the rechargeable battery 8. In this case, the resonance frequencies of the power transmission coil 4a and the power receiving coil 4b are set to be the same.
In the condition of wirelessly transmitting and receiving power by the electromagnetic induction-type method or the magnetic field resonance-type method, if the power receiving coil unit is not appropriately disposed with respect to the power transmission coil unit, it is difficult to transfer power efficiently. Particularly, when a secondary battery that is charged by the power receiver 2 is mounted to portable equipment, a shielding member (including a wave absorber) for minimizing the influence of electromagnetic waves may be inserted between the power receiving coil unit and the secondary battery pack. In such a case, when the portable power receiving coil unit is placed upside down, the shielding member is interposed between the power transmission coil unit and the power receiving coil unit, which results in a significant reduction in the transfer efficiency and makes power transfer difficult.
Therefore, JP 2010-207017A discloses a charging system including a front/back discrimination unit (using a magnetic sensor) that detects whether the surface suitable for power reception of a power receiving coil unit faces properly a power transmission coil unit that supplies power, so that if the surface does not properly face the power transmission coil unit, the user is warned.
In the art disclosed in JP 2010-207017A, if the surface suitable for power reception of a power receiver does not properly face a power transmitter that supplies power, the user is merely warned by means of a sound or the like and eventually has to bring the surface back to the proper posture, making the operation troublesome. Moreover, in the art disclosed in JP 2010-207017A, a magnetic sensor is provided on the power receiver side for the front/back discrimination, and therefore, it is difficult to provide such function other than the charging function in the power receiver if the power receiver is small (e.g., an in-the-ear hearing aid).
If the size of the power receiving coil unit is small relative to that of the power transmission coil unit, a reduction in the power transfer efficiency, a reduction in the possible power transfer distance, and the like may occur. Furthermore, also when the coupling coefficient or the like has changed due to a change in the state such as a change in the distance between the power transmission coil unit and the power receiving coil unit, it is desired to provide an adjusting circuit in the power receiver in order to match the resonance frequencies. However, as described above, it is difficult to provide any additional function in the power receiver other than the charging function.